1. Field
The present application generally relates to the design of an etch process control system to measure a structure formed on a workpiece, and, more particularly, to a method of monitoring an etch process to meet two or more etch stage measurement objectives.
2. Related Art
During semiconductor processing, an etch process, for example, a reactive ion etch (RIE) process, is employed for etching fine line patterns in a workpiece such as a silicon substrate or wafer. RIE involves positioning a wafer in a chamber that contains a plasma. The plasma contains gases that are dissociated in a radio frequency field so that reactive ions contained in the gases are accelerated toward the wafer surface. The reactive ions combine chemically with material on the wafer surface. During the etch process, one or more layers of material can be removed. Endpoint determination or detection is used in controlling etch processes.
As the one or more layers of material are etched, the volatile etch products are incorporated into the plasma. As the RIE process approaches the interface or end of the layer being etched, the amount of volatile etch product found in the plasma decreases. The amount of volatile etch product in the plasma can be tracked to determine the endpoint of the ME process. One of the species can be tracked such as one of the etchant gases used to etch a layer of material. As the layer is etched, the reactive species will be used up and relatively low concentrations of the reactive species will be found in the plasma. As more and more of the layer or layers are used up, the reactive species will be found in the plasma in increasingly higher concentrations. A time graph of the optical emissions from such a reactive species will show an increase in intensity as the layer is etched away. Tracking the intensity of a wavelength for a particular species using optical emission spectroscopy (OES) can also be used for endpoint determination or etch process control such as an RIE process.
Typically, OES has been used to track the amount of either volatile etch products or reactive species as a function of film thickness. These techniques examine emissions from either the volatile etch products or reactive species in the plasma. For example, during an RIE process, plasma discharge materials, such as etchant, neutral, and ions in the plasma, are continuously excited by collisions. An optical emission spectrometer diffracts emissions into its component wavelengths. A specific wavelength can be associated with a particular species, and this association can be used to detect an etch endpoint. However, such specific wavelength information is typically unavailable, and it is difficult to select an appropriate wavelength to use for accurate etch endpoint determination due to numerous possibilities for emissions. The optimal wavelength or wavelengths are not readily known due to number of variables in a typical etch process. For example, the OES spectrum for a typical RIE etch can be composed of hundreds of wavelengths in the visible and ultraviolet bands.
In addition, there is a trend towards using high-density plasma sources to replace RIE. Examples include use of a high-density, inductively-coupled plasma (ICP). Another example is in the use of electron cyclotron resonance (ECR), which differs from RIE in plasma formation. Generally, ECR operates at a lower pressure than a regular RIE system, and is, therefore, able to etch finer line trenches. Comparison studies of the emissions from high-density ICP, ECR and RIE plasmas show emphasis on different species and different wavelengths for similar input gas compositions. The data accumulated from RIE emissions may not be applicable to high-density ICP emissions and ECR emissions.
Prior art techniques for determining an endpoint in an etching process using OES spectra are described, for example, in U.S. Pat. No. 5,288,367, to Angell et al., entitled “END-POINT DETECTION”, in U.S. Pat. No. 5,658,423, to Angell et al., entitled “MONITORING AND CONTROLLING PLASMA PROCESSES VIA OPTICAL EMISSION USING PRINCIPAL COMPONENT ANALYSIS”. These prior art techniques typically entail selecting one wavelength to be used for signaling an etch endpoint, however. A prior art technique for performing process control by statistical analysis of the optical spectrum of a product produced in a chemical process is described, for example, in U.S. Pat. No. 5,862,060, to Murray, Jr., entitled “MAINTENANCE OF PROCESS CONTROL BY STATISTICAL ANALYSIS OF PRODUCT OPTICAL SPECTRUM” ('060). The '060 patent describes measuring the optical spectrum of each member of a calibration sample set of selected products, determining by Principal Component Analysis (PCA) or Partial Least Squares, (PLS).
Problems with aforementioned techniques for determining an endpoint in an etching process using PCA applied to OES spectra includes the uncertainty of the number of components to use in the PCA analysis. More principal components used, the better PCA approximates the system being analyzed, but more computer resources are needed. Moreover, determining the optimal number of PCA principal components is also time intensive and uses a lot of resources.
Furthermore, state-of-the art OES systems are capable of collecting a plurality of wavelengths of optical emission spectra emanating from the glow discharge of gases in a plasma etch chamber. These wavelengths can be associated with the specific chemical species generated from entering reactant gases, can result from gas phase reactions as well as reactions on the wafer and chamber surfaces. The wavelengths of the optical emission spectra can also shift as the surface composition of the wafer shifts from a steady-state etch to the complete removal of the etched material. Detection of this shift allows for etch endpoint determination, indicating the completion of the required etch and also can allow for termination of the etch process before over-etching occur. However, the number of OES frequencies or wavelengths available to determine an etch endpoint creates the problem of a complex and time consuming selection of the appropriate OES wavelengths.
Endpoint in an etching process can also be determined using a broadband light source process; endpoint detection are described, for example, in U.S. Pat. No. 6,979,578 to Venugopal, entitled “PROCESS ENDPOINT DETECTION METHOD USING BROADBAND REFLECTOMETRY”, using multiple optical signals obtained from multiple measurement location are described in U.S. Patent Application No. 2006/0012796 to Saito et al., entitled “PLASMA TREATMENT APPARATUS AND LIGHT DETECTION METHOD OF A PLASMA TREATMENT”, and using at least two optical components for endpoint detection for photomask etching in U.S. Patent Application No. 2006/0014409 to Grimbergen entitled “ENDPOINT DETECTION FOR PHOTOMASK ETCHING”. As mentioned above, there is the problem of selecting the appropriate metrology tools that will work for a semiconductor application or range of applications using the aforementioned techniques. Furthermore, there is a need to optimize the selection of wavelength or wavelengths, specific optical metrology tools, and algorithms for extraction of etch stage measurement to meet measurement monitoring objectives. Moreover, in an integrated metrology fabrication cluster, there is a need to complete etch measurement monitoring in real-time and extraction of etch stage data to meet targeted time ranges. In other etching applications, there is also a need to ensure the repeatability and reproducibility of the extracted etch stage data from the etch stage measurements.
The present invention is directed to minimizing the effects of one or more of the problems discussed above. With increased requirement for throughput, decreasing size of the structures, and requirement for lower cost of ownership, there is greater need to optimize design of etch stage measurement systems to meet one or more etch stage measurement objectives.